Several approaches to chemical identification are based on the recognition that ion species have different ion mobility characteristics under different electric field conditions at atmospheric pressure. These approaches include time-of-flight Ion Mobility Spectrometry (IMS) and differential mobility spectrometry (DMS), the latter also known by other names such as field asymmetric ion mobility spectrometry (FAIMS). Atmospheric-pressure chemical ionization enables these identification processes (including radioactive, ultraviolet and electrospray ionization, for example). In a conventional IMS device, a weak DC field gradient is established between an upstream electrode and a downstream collector electrode and then an ionized sample is released into the DC field. The ionized sample flows toward the collector electrode. Ion species are identified based on the time of flight of the ions to the collector. The DC field is weak where ion mobility is constant.
A typical DMS device includes a pair of opposed filter electrodes defining an analytical gap between them in a flow path (also known as a drift tube or flow channel). Ions flow into the analytical gap. A compensated high-low varying asymmetric RF field (sometimes referred to as a filter field, a dispersion field or a separation field) is generated between the electrodes transverse the ion flow in the gap. Field strength varies as the applied RF voltage (sometimes referred to as dispersion voltage, separation voltage, or RF voltage) and size of the gap between the electrodes. Such systems typically operate at atmospheric pressure.
Ions are displaced transversely by the DMS filter field, with a given species being displaced a characteristic amount transversely toward the electrodes per cycle. DC compensation is applied to the electrodes to compensate or offset the transverse displacement generated by the applied RF for a selected ion species. The result is zero or near-zero net transverse displacement for that species, which enables that species to pass through the filter for downstream processing such as detection and identification. Other ions undergo a net transverse displacement toward the filter electrodes and will eventually undergo collisional neutralization on one of the electrodes.